1. Field of the Invention
The present invention relates generally to the field of radio communications, and more particularly to the field of eliminating DC offset in a high frequency radio signal.
2. Description of the Related Art
In most radio communication systems, or any other system having a transmitted signal mixed with a carrier frequency, there is a carrier frequency offset between the receiver and the transmitter due to the use of different oscillators in the communicating devices. Due to imperfect matching and many other radio design trade-offs, there often exists a non-zero DC offset at the receiver. This DC offset resides precisely at the DC (frequency=0) in the received spectrum. After the transmitted signal has been down-converted to the baseband (processing frequency) in the receiver, the signal generated by the transmitter and the channel centered around the carrier frequency will be down-converted to a spectrum centered at a frequency that is equal to the carrier frequency offset between the transmitter and the receiver, denoted foffset.
The appearance of DC offset is illustrated in FIG. 1, where the receiver DC offset appears at frequency=0 and the carrier leak from the transmitted signal appears at frequency=foffset. The carrier leak signal is a narrow band signal transmitted at the carrier frequency, caused primarily by imperfect digital-to-analog converters (DACs), and feed-through in the mixers. It is very difficult using known techniques to eliminate the receiver DC offset in the presence of the down-converted transmitted signal.
Most available DC offset cancellation algorithms are based on low-pass filtering (LPF), notch filtering, or averaging. For example, if an accumulator is used at the receiver to accumulate samples of the received signal in the time domain, an estimate of the receiver DC offset can be obtained. The notch filter operates in a similar fashion in that it “filters” the DC component out of the received spectrum, as illustrated in FIG. 2. However, as shown in FIG. 2, the DC offset estimated from averaging or notch-filtering is not perfect, as it cannot distinguish the transmitted signal from the receiver DC offset. This may result in the loss of signal energy.
DC offset is also eliminated in the analog domain, by correcting the frequency offset first (calibrating) and then using an AC coupling circuit. However, analog solutions are generally more costly and less robust due to circuit fabrication process variations. There is therefore a need in the art for a DC offset solution that circumvents the need for filtering or analog calibration.